This is an application for a NIDA senior scientist award. The primary goal of my research is to understand the mechanisms underlying the acute and chronic effects of cannabinoids (the principal psychoactive constituents of cannabis). My studies are focused on the function and regulation of the CB1 (neuronal) and CB2 ("peripheral") cannabinoid receptors. Cannabinoids, acting at CB1 receptors are potent modulators of ion channel function. Consistent with these actions, efficacious cannabinoids decrease neuronal excitability and inhibit neurotransmitter release. Interestingly, endogenous cannabinoids acting via CB1 receptors inhibit neurotransmission and mediate several forms of short and long-term neuronal plasticity-in part through non-ion channel signaling pathways. CB2 receptors, whose activation is devoid of apparent psychoactivity, have recently been shown to be involved in processes as diverse as analgesia, bone growth, and atherosclerosis, and are receiving intense scrutiny as potential therapeutic targets. Over the next five years we will continue our studies at the molecular and genetic level, emphasizing the following specific aims. Is behavioral tolerance to cannabinoids due to CB1 receptor phosphorylation? Is it possible to identify CB1 and CB2 agonists that cause little desensitization? What role does dimerization have in the signaling of CB1 and CB2 receptors? What are the determinants of the speed of CB1 receptor signaling? What is the role of GPR55, a third cannabinoid receptor, in cannabinoid action? Accomplishing these goals will significantly advance our understanding of the cellular actions of cannabinoids. To accomplish these ambitious goals I have assembled a group of collaborators and consultants whose expertise complements that currently in place in my laboratory. Three facts underscore the importance of having a solid understanding of cannabinoid receptor function at the molecular level. 1. Chronic cannabis use is a significant social issue. 2. We need a firm understanding of the cellular effects of cannabinoid receptor activation to interpret the possible benefits of their therapeutic manipulation. 3. We know little about the physiological role of endogenous cannabinoids in healthy or diseased tissues.